A chromatographic analyzer is an analytical instrument used to separate in time and individually detect the constituents of a sample to be analyzed. The chromatographic analyzer typically includes an analytical column through which at least a carrier fluid stream is passed continuously. The sample to be analyzed is introduced into the carrier fluid stream and is thus carried through the analytical column. The sample constituents are carried through the analytical column at different rates and in this manner the sample constituents are separated in time.
A detector is employed to detect the separated constituents and the detector output signal is typically recorded as a function of time by a recorder to produce a chromatogram. As each sample component is eluted from the column, the component typically produces an increase in the detector output signal amplitude which appears as a peak or spike on the chromatogram.
There are many different types of chromatographic analyzer detectors. Generally, chromatographic analyzer detectors have a sample side and a reference side. Carrier fluid flows through the reference side. Carrier fluid containing an injected sample flows through the sample side. Some characteristic such an optical absorbance or transmission, electrical conductivity, or refractive index of the carrier alone is compared to the same characteristic of the carrier containing the sample to produce a difference signal. The detector output signal amplitude is representative of the difference signal and therefore permits analysis of the individual components of the sample.
An optical absorbance detector achieves chemical analysis by absorption spectroscopy in which radiation of various wavelengths is passed though a material and the radiation transmitted, or absorbed, is measured to determine material identity and concentration. Where analysis only of components absorbing at a specific wavelength is desired, this can be accomplished by using light sources emitting only specific wavelengths and/or interference filters to pass only the desired wavelengths.
One source of error in optical absorbance detectors can be error arising from refractive index effects. The refractive index effects can arise from temperature changes produced by absorption of light in optical flow cells of optical absorbance detectors. Such refractive index effects are disclosed, for example, in U.S. Pat. No. 4,019,372 which also discloses temperature control means for reducing refractive index error. Refractive index effect error can also arise from the optics of the optical absorbance detector. Accordingly, means for reducing refractive index error whether arising from temperature changes or from the optics of the optical absorbance detector are highly desirable.
Other sources of error can arise because of stray light effects and/or light crossover or crosstalk between a reference light beam and a sample light beam in an optical absorbance detector as well as from temperature variation within the optical absorbance detector which can influence electrical signals produced by the detector.
Use of chromatographic analyzers having optical absorbance detectors for continuous process monitoring and/or control places stringent constraints upon the process detector. The process detector must be suitable for use under conditions found in process applications. These conditions can include temperature extremes, the presence of explosive gases, and the like. The process detector must be rugged and capable of safe operation under such conditions and at the same time provide the sensitivity, board linear dynamic range, and long term stability necessary for process control.
Accordingly, an object of the present invention is an optical absorbance detector having an extended linear dynamic range. Another object is a ruggedized optical absorbance detector which can provide the sensitivity, linear dynamic range, and long term stability required for process applications. A further object is an optical absorbance detector capable of selectable multi-wavelength operation which can provide the sensitivity, linear dynamic range, and long term stability required for process applications. Yet another object is an optical absorbance detector with minimal refractive index error. Another object is an optical absorbance detector having minimal error resulting from stray light or light crossover. Another object is an optical absorbance detector having minimal error resulting from temperature variation. Yet another object is an optical absorbance detector with minimal refractive index error. Another object is an optical absorbance detector having minimal error resulting from stray light or light crossover. Yet other objects of the present invention include chromatographic analyzer systems utilizing such optical absorbance detectors. Other objects and advantages will be apparent to one of ordinary skill in the art from the following disclosure and the drawings in which: